In networks such as the Internet and the like, there are cases in which delay of data transmission, loss of packets and jitters of transmission delay occur due to transmission band, channel quality and the like. For this reason, when streaming data obtained by compressing continuous media such as video and sound information is transmitted and reproduced in a continuous way, there occur cases where data transmission quality is corrupted so that it is difficult to reproduce it smoothly.
That is, a network can be considered to be a combination of a queue of a limited length and a channel. If an amount of data equal to or greater than the bandwidth of the channel is attempted to flow, the data is accumulated in the queue, and the delay taken for transmission increases. A further continuation of flowing data equal to or greater than the bandwidth overflows the queue and packets are lost. Also, in use of a channel low in transmission quality there occur cases that packets are lost without regards to the amount of data, the order of packets is changed, and transmission delay fluctuates.
To deal with, for example, as disclosed in Patent document 1 referred to later, the encoding rate is controlled in order to perform efficient data transmission by suppressing quality degradation on the data receiving side.
FIG. 11 is a block diagram showing a conventional communication system. For convenience of illustration, the system is divided into parts (a) and (b) at point A. The transmission side includes a camera 101, a real-time encoder 102 and a transmission controller 103 while the reception side includes a reception controller 111, a real-time decoder 112, a reproducing display 113. The transmission side and the reception side are connected by a network 105.
On the transmission side in (a) of the same figure, video information input from camera 101 is real-time encoded by real-time encoder 102 at a transmission bit rate designated by transmission controller 103. The transmission bit rate in real-time encoder 102 is designated by bit rate feedback information from transmission controller 103.
In this data format for transmission, in order to enable the order and reproduction timing of transmitted packets to be recognized on the reception side, a serial number is allotted to every packet and time stamps representing the reproduction timing of the data included in each packet are also added. As a format for this purpose, RTP (Real-time Transport Protocol) has been widely used. Also, in order to control the transmission based on RTP, RTCP (RTP Control Protocol) has been widely used. Though the present invention as well as the prior art is not limited to RTP/RTCP, for simplicity the following description will be made referring to a system that is presumed based on RTP/RTCP which can be widely understood.
Video data packets transmitted through network 105 are received by reception controller 111 on the reception side in (b) in the figure. In reception controller 111, based on the serial numbers attached to the received video data packets, irregular reception status of video data packets such as loss of video data packets, change in the order of arrival, repeated reception and the like is detected. From the video data packets which are determined to have arrived in time for reproduction, video data stored in their payloads is picked up. Video data packets which are determined to be late for their reproduction timing are discarded. The video data is transferred together with time stamps to real-time decoder 112, where it is decoded in real time and reproduced and displayed on reproducing display 113 in accordance with the time stamps.
Other than video data packets, transmission controller 103 also sends out SR (Sender Report) packets (to be called SR packets, hereinbelow) for carrying control information in order to enable measurement of quality parameters of the network to determine the status of congestion. The SR packet is sent out periodically from transmission controller 103 to reception controller 111 on the reception side in FIG. 1(b). The format of a SR packet includes time information (transmission time stamp) on the time when it was transmitted.
On the other hand, when receiving a SR packet, reception controller 111 sends out a RR (Receiver Report) packet (to be called RR packet hereinbelow) for carrying control information to transmission controller 103. The format of the RR packet includes the transmission time stamp information included in the received SR packet and information on the lapse time from when the reception side received the SR packet until the RR packet is transmitted.
In transmission controller 103 that has received the RR packet, it is possible to determine the outgoing and returning transmission delay (Round Trip Time) of the control packets (SR packet and RR packet) by the following equation (1)Trnd=(Trcv−Tsnd)−Tstay  (1)where    Trnd: round trip time of control signal packets;    Trcv: time at which the transmission controller received a RR packet;    Tsnd: time at which the SR packet that corresponds to the RR packet received by the transmission controller was sent out (transmission time stamp information stored in the RR packet); and    Tstay: time from when the reception controller received a SR packet until it sends out the RR packet (lapse time information stored in the RR packet).
The format of the RR packet also stores network congestion information such as the packet loss ratio relating to video data packets received by reception controller 111 and jitter information representing fluctuation of transmission delay.    Patent Document 1:
Japanese Patent Application Laid-open 2003-244695.